1. Field of the Invention
The present invention relates to an improved method and device for regenerating, from signals received on a transmission channel, a clock signal marking the transmission, on the channel, of digitized and coded data.
2. Description of the Prior Art
In data transmission using clock signals, the digitized data to be transmitted are coded from a predetermined number of different voltages of constant amplitude. 4, 8, 16 or coding symmetrical voltages are for example selected on either side of the zero voltage. With 2.sup.n levels for example, n bits can thus be transmitted simultaneously and the transmission rate of a transmission channel can therefore be increased. An application of transmission on a cable using a multilevel coding is described for example in the Assignee's French Patent 2,675,974 for optimizing the signal transfer rates on transmission cables.
The Assignee's French Patent 2,683,411 (EP-541,431) describes a process and a device for restoring a clock signal marking the transmission of signals received on a transmission channel, notably for the radio transmission of seismic signals.
In order to restore the clock signal, the transition times of the signals are detected with precision through an analysis of a series of samples taken from the signals for at least a fraction of the clock period. The amplitude differences between the various successive samples are detected and the obtained difference configuration is compared with standard configurations observed on the transmission channel by means of previous tests and stored in a memory. If the similarity between the measured difference configuration and one of the standard configurations is sufficient, the window is correctly centered. If it is not, a local clock is used for resynchronization.
This prior method works correctly in all the cases where the transitions between the successive levels are sufficiently marked. In the opposite case, which notably occurs when the passband of the transmission channel is incompatible or poorly compatible with the desired transmission rate, the transition zones are too degraded for the configurations of samples taken in each "window" to correspond to a stored standard configuration. The resynchronization of a local clock on the basis of the comparisons performed is therefore more uncertain.
In geophysical exploration operations intended to connect downhole servicing equipments: measuring instruments, well sondes, etc, to a surface control and recording station, transmission cables with a limited passband are generally used, such as multi-conductor electrocarrying cables.
By using the optimization process described in the Assignee's French Patent 2,675,974, the transmission rate of the transmission lines can be optimized in this type of cable. To that effect, correction circuits are placed at the end of a line so that the transfer function of the corrected line corresponds at least within a certain frequency interval to that of a reference filter such as a Bessel filter for example. The presence of these correction circuits allows the transmission rate of the transmission lines to be improved considerably.
However, it has been noticed during transmission tests on this type of cable, either because the transmission rate is not compatible with the natural passband of the lines, or because the correction applied thereto is not optimal, the signals exhibit a horizontal instability (jitter) as they approach zero, as shown in FIG. 1, which makes the transitions between different levels hardly detectable, and their vertices are rounded in the shape of sinusoid vertices, however with a relatively stable position. FIG. 1 highlights this phenomenon.
On the other hand, it has been observed that it is easier to discern when the signals go through their maximum amplitude in a relatively narrow sampling window.
It is well-known that a local clock can be synchronized with the clock that marks the transmission of a signal by detecting the times when the signal derivative with respect to time becomes equal to zero and by adjusting the local clock by means of a locking loop.
However, in the case when the received signal is of the multilevel type, such a detection can lead (see FIG. 3) to an erroneous adjustment of the local clock to intermediate times where the derivative changes sign (points m in FIG. 3) and not to the really representative vertices (vertices M).